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Wed, 11 Dec 2019 12:52:48 +0000 (UTC) Subject: Re: [PATCH v6 28/42] stream: Deal with filters From: Max Reitz To: Kevin Wolf References: <20190809161407.11920-1-mreitz@redhat.com> <20190809161407.11920-29-mreitz@redhat.com> <20190913141653.GH8312@dhcp-200-226.str.redhat.com> <0da03f2f-e7ca-1aad-f156-bbd8a0e9dbc7@redhat.com> Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <1b849809-ac7e-091b-4987-b3ee9383cb15@redhat.com> Date: Wed, 11 Dec 2019 13:52:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <0da03f2f-e7ca-1aad-f156-bbd8a0e9dbc7@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="9OTqtuTDjRKDzFW2d6NuhyqCznjmfd9fP" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --9OTqtuTDjRKDzFW2d6NuhyqCznjmfd9fP Content-Type: multipart/mixed; boundary="k9E2PD3MvLINLOU3ZAjOZBDpzVHbWX27T" --k9E2PD3MvLINLOU3ZAjOZBDpzVHbWX27T Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 16.09.19 11:52, Max Reitz wrote: > On 13.09.19 16:16, Kevin Wolf wrote: >> Am 09.08.2019 um 18:13 hat Max Reitz geschrieben: >>> Because of the recent changes that make the stream job independent of >>> the base node and instead track the node above it, we have to split tha= t >>> "bottom" node into two cases: The bottom COW node, and the node directl= y >>> above the base node (which may be an R/W filter or the bottom COW node)= . >>> >>> Signed-off-by: Max Reitz >>> --- >>> qapi/block-core.json | 4 ++++ >>> block/stream.c | 52 ++++++++++++++++++++++++++++---------------- >>> blockdev.c | 2 +- >>> 3 files changed, 38 insertions(+), 20 deletions(-) [...] >>> + if (bdrv_freeze_chain(bs, above_base, errp) < 0) { >>> return; >>> } >> >> Hm... This feels odd. There are two places where stopping to freeze the >> chain would make obvious sense: At base, like we originally did; or at >> base_overlay, like we (intend to) do since commit c624b015, because we >> say that we don't actually mind if the user replaces the base image. I >> don't see how stopping at the first filter above base makes sense. >> >> So should this use bottom_cow_node/base_overlay instead of above_base? >=20 > I suppose I thought =E2=80=9CBetter be safe than sorry=E2=80=9D. >=20 >> You couldn't use StreamBlockJob.above_base any more then because it >> could change, but you also don't really need it anywhere. It's only used >> for unfreezing (which would change) and for finding the base (you can >> still find bdrv_backing_chain_next(s->base_overlay)). I guess this would >> even be a code simplification. >=20 > Great, I=E2=80=99ll see to it. On second thought (yes, I know, it=E2=80=99s been a couple of months...) I= =E2=80=99m not so sure. If @base is a filter, then bdrv_backing_chain_next(s->base_overlay) will not return it. So then the filter will be dropped, but that probably isn=E2=80=99t what the user intended. (In fact, the block-stream doc says =E2=80=9CWhen streaming completes the i= mage file will have the base file as its backing file.=E2=80=9D) So now this gets hairy. We do want exactly @base as the backing file unless the user changed the graph. But how can we detect that and if it hasn=E2=80=99t changed find @base again? What this patch did in this version worked because the graph was frozen until @above_base. Alternatively, we could store a pointer to @base directly (or its node nmae) and then see whether there is some node between s->base_overlay and bdrv_backing_chain_next(s->base_overlay) that matches that at the end of streaming. Well, actually, a pointer won=E2=80=99t work because of course maybe that n= ode was deleted and the area is now used for an unrelated node that the user doesn=E2=80=99t want as the new backing file. The node name could actually work, though. I mean, if there is some node in the immediate backing filter chain of base_overlay with that node name after streaming, it does make sense to assume that the user wants this to be the backing file; regardless of whether that=E2=80=99s exa= ctly the same node as it was at the start of streaming. But we now also have to think about what to do when there is no node with such a node name. Should we keep all filters below base_overlay? Or should we drop all of them? I actually think keeping them is the safer choice... Max --k9E2PD3MvLINLOU3ZAjOZBDpzVHbWX27T-- --9OTqtuTDjRKDzFW2d6NuhyqCznjmfd9fP Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3w5p4ACgkQ9AfbAGHV z0ByDgf/eyFsEiHG8ALOsbHOEd74vOPo1bCUxxAneU8TUzdw41CJPaynepIJICwc fF3BuA1mdiw+FgcTjMfoNkmSfDdyhfhaKVSNaN1SHvuZLR89JmM0GaL+Zjfw7A29 46b0jjSlB/Lr1wYlSy8q1YKBn+6GE+eaI8/5XJ9J79unPq0IFWlBBmlTurD680Ui n5hA2vnLZy1xCBYUpj70TxIaLc0/0ca7rIHN1UrgoR6he2SIuFBn66gPftBhDT+k +btu3PB61esB6Y5bD0rHY5zYMqIgdWQFT4KzYYbfmhCtfTDmVlUGMl1oinMisKwr xgkUmPBD7ylOn8y70Mp9dqUoAqI43g== =Zg7l -----END PGP SIGNATURE----- --9OTqtuTDjRKDzFW2d6NuhyqCznjmfd9fP--